Many commercially valuable agricultural crops are prone to infection by plant viruses. These viruses are capable of inflicting significant damage to a crop in a given season, and thus can drastically reduce its economic value. The reduction in economic value to the farmer in turn results in a higher cost of goods to ultimate purchasers. Several published studies have been directed to the expression of plant virus capsid proteins in a plant in an effort to confer resistance to viruses. See, e.g., Abel et al., Science 232:738-43 (1986); Cuozzo et al., Bio/Technology 6:549-57 (1988); Hemenway et al., EMBO J. 7:1273-80 (1988); Stark et al., Bio/Technology 7:1257-62 (1989); and Lawson et al., Bio/Technology 8:127-34 (1990). However, the transgenic plants exhibited resistance only to the homologous virus and related viruses, but not to unrelated viruses. Kawchuk et al., Mol. Plant-Microbe Interactions 3:301-07 (1990), disclose the expression of wild-type potato leafroll virus (PLRV) coat protein gene in potato plants. Even though the infected plants exhibited resistance to PLRV, all of the transgenic plants that were inoculated with PLRV became infected with the virus and thus disadvantageously allowed for the continued transmission of the virus such that high levels of resistance could not be expected. See U.S. Pat. No. 5,304,730.
Lodge et al., Proc. Natl. Acad. Sci. USA 90:7089-93 (1993), report the Agrobacterium tumefaciens-mediated transformation of tobacco with a cDNA encoding wild-type pokeweed antiviral protein (PAP) and the resistance of the transgenic tobacco plants to unrelated viruses. PAP, a Type I ribosome-inhibiting protein (RIP) found in the cell walls of Phytolacca americana (pokeweed), is a single polypeptide chain that catalytically removes a specific adenine residue from a highly conserved stem-loop structure in the 28S rRNA of eukaryotic ribosomes, interfering with elongation factor-2 binding and blocking cellular protein synthesis. See, e.g., Irvin et al., Pharmac. Ther. 55:279-302 (1992); Endo et al., Biophys. Res. Comm., 150:1032-36 (1988); and Hartley et al., FEBS Lett. 290:65-68 (1991). The observations by Lodge were in sharp contrast to previous studies, supra, which reported that transgenic plants expressing a viral gene were resistant to that virus and closely related viruses only. See also Beachy et al., Ann. Rev. Phytopathol. 28:451-74 (1990); and Golemboski et al., Proc. Natl. Acad. Sci. USA 87:6311-15 (1990). Lodge also reports, however, that the PAP-expressing tobacco plants (i.e., above 10 ng/mg protein) tended to have a stunted, mottled phenotype, and that other transgenic tobacco plants that accumulated the highest levels of PAP were sterile.
Hence, a need remains for a means by which to confer broad spectrum virus resistance to plants which overcomes the problems associated with known methods, and particularly which would require a minimum number of transgenes, the expression of which would not cause plant cell death or sterility.